1. Field of the Invention
The present invention relates to a transmission component having a long rolling contact fatigue life, an increased anti-crack strength, or a reduced secular variation in dimension, and to a method of manufacturing the same and a tapered roller bearing.
2. Description of the Background Art
To increase a bearing's components in life, a thermal treatment is performed. Specifically, for example, in quenching the components they are heated in an ambient RX gas with ammonium gas further introduced therein to carbo-nitride their surface layer portion, for example as disclosed in Japanese Patent Laying-Open Nos. 8-4774 and 11-101247. This carbonitriding process can harden the surface layer portion and generate retained austenite in a microstructure to provide increased rolling contact fatigue life.
The above-mentioned carbonitriding process is a process to diffuse carbon and nitrogen. This requires a high temperature maintained for a long period of time. As such, for example a coarsened structure results and increased anti-crack strength is hardly obtained. Furthermore, as more austenite is retained, secular dimensional variation rate increases, which is also a problem in this carbonitriding process.
Against rolling fatigue, an increased life can be ensured, an enhanced anti-crack strength provided and an increased secular dimensional variation avoided by relying on designing a steel alloy to provide an adjusted composition. Relying on designing the alloy, however, increases source material cost disadvantageously.
Future bearings in a transmission will be used in environments exerting large loads at high temperatures. In addition, they will be required to be employed in a downsized transmission. Therefore, the bearings will be required to be operable under larger loads at higher temperatures than conventional. As such, there is a demand for a bearing having large strength, long life against rolling contact fatigue, and large anti-crack strength and dimensional stability.